Generally defined, the term "process variable" refers to a physical or chemical state of matter or conversion of energy. Examples of process variables include pressure, temperature, flow, conductivity, pH, and other properties. The term "process measurement" refers to the acquisition of information that establishes the magnitude of process quantities. Pressure is considered a basic process variable in that it is used for the measurement of flow (the difference of two pressures), level (head or back pressure), and even temperature (fluid pressure in a thermal system).
An industrial process transmitter is a transducer that responds to a measured variable with a sensing element and converts the variable to a standardized transmission signal, e.g., an electrical or optical signal or air pressure, that is a function of the measured variable. Industrial process pressure transmitters are used with the pressure measurement of an industrial process such as slurries, liquids, vapors and gasses in chemical, pulp, petroleum, gas, pharmaceutical, food, and other fluid processing plants. Industrial process transmitters are often placed near the process fluids, or in field applications. Often, these field applications are subject to harsh and varying environmental conditions that provide challenges for designers of such transmitters.
The sensing element in many pressure transmitters is a capacitance sensor that includes a deflectable sensing diaphragm ("diaphragm") and two capacitor electrodes. A first type of sensing element includes a diaphragm, which is a conductive stretched membrane that deflects in response to pressures applied on both sides of the diaphragm, and two capacitor electrodes, one on each side of the diaphragm. A dielectric fill-fluid is used between the capacitor plates and the diaphragm. The fill fluid, used with an isolating diaphragm interfacing with the process fluid, prevents the process fluid, which at times can be harsh, corrosive, dirty or contaminated, from interacting with the components of the sensing element and perhaps damaging the components. A first capacitor electrode, on one side of the diaphragm, coupled with the conductive diaphragm forms a first capacitor. A second capacitor electrode, on the opposite side of the diaphragm, coupled with the diaphragm forms a second capacitor. The capacitance of each capacitor changes in proportion to the inverse of the distance between the capacitor plate and the diaphragm. Thus, the capacitance of each capacitor changes as the diaphragm deflects in response to the applied pressures. The amount of deflection is related to the difference between the two applied pressures, or differential pressure. The differential capacitance between each capacitor plate and the conductive diaphragm is detected and is used to provide the standardized transmission signal, which is related to differential pressure.
The sensing element is particularly adapted to detect diaphragm deflection in a process field environment. The approximate relationship between the capacitance, C, and the distance between one of the capacitor plates, X, is C=.epsilon.K/X, where .epsilon. is the permittivity of the fill-fluid and K is a constant depending on several factors such as the geometry of the sensing element. The permittivity .epsilon. of a typical fill-fluid is sensitive to changes in a process field environment. The permittivity .epsilon. typically varies by approximately 15% over typical temperature ranges of a fill fluid in process field environments. The sensing element with two opposing capacitors is configured such that the output is generally independent of a varying permittivity. The two capacitors in the sensing element generally provide an output related to the ratio (C.sub.1 -C.sub.2)/(C.sub.1 +C.sub.2), where C.sub.1 is representative of the capacitance of the first capacitor and C.sub.2 is representative of the capacitance of the second capacitor in the sensing element. The permittivity .epsilon. in the numerator cancels the permittivity in the denominator of this ratio. Accordingly, the sensing element is generally insensitive to temperature changes of a fill fluid in a process field environment.
A second type of capacitive sensing element is known, but unlike the first type of sensing element described above, the second type of sensing element is not suited for measuring differential pressure. Instead, the second type of sensing element is used to measure absolute pressure. The second type of sensing element has two capacitor plates, forming two capacitors, on one side of the diaphragm, rather than on opposite sides of the diaphragm. The second type of sensor does not use a fill-fluid. Absolute process pressure is applied to the sensor diaphragm on the side opposite the electrodes. The second type of sensor includes a ceramic substrate to position two capacitor electrodes the same distance from the undeflected conductive diaphragm. The two capacitor electrodes are positioned in a plane on one side of the diaphragm in order to compensate for undesired motion of the ceramic substrate. The sensor output is related to the difference of the capacitances. The sensor detects curvature of the parabolically deflected diaphragm, and the sensor output cancels or disregards piston-like movement of the ceramic substrate that typically occurs as a result of temperature changes. The second type sensor is not suited for use with a fill fluid on the electrode side of the diaphragm because the transfer function that compensates for piston-like motion of the substrate is unable to compensate for changes in the permittivity of that fill fluid as the, temperature changes.
The ability to detect the curvature of the deflected diaphragm in a field process environment has advantages over merely the ability to detect the amount of deflection of the diaphragm. Ideally, the displacement of the diaphragm is proportional to the differences in the pressures applied to both sides of the diaphragm. Unfortunately, the diaphragm does not deflect in an ideal manner. Due to physical forces inherent in stretched membranes, diaphragms are often deformed after they have been deflected. This deformation can be described as an "offset," where the very edges of the diaphragm are bent in such a way so that the flat portion of the diaphragm is spaced closer to one of the electrodes than the it is in an ideal spacing, i.e., the edges are bent so as the flat central portion protrudes toward one or the other of the electrodes. The offset is not detected by merely sensing the diaphragm deflection in the sensing element of the first type, and this offset causes inaccurate readings of the process pressure. These inaccurate readings have gone uncompensated for at least two reasons. First, there was a lack of appreciation of the source of these errors and the resulting diaphragm offset. Second, the sensing elements and transmitters currently available are unable to separate curvature from offset in a measurement of deflection, and still compensate for varying permittivity due to changing temperatures in a process field environment.